A recent article in the journal Blood found evidence that a certain, relatively uncommon, blood type may be associated with resistance to HIV infection. This blood type, which is not part of the more commonly discussed A/B/O or Rh groupings, is characterized by the amount of a protein known as Pk present on blood cells. In this small study, the blood cells of those individuals who had higher levels of Pk were found to be more resistant to infection by HIV. This genetic factor could explain some of the population variance in susceptibility to HIV and AIDS. This is not the first genetic factor that has been found to be associated with susceptibility to HIV. A mutation in the protein CCR5, that is found most commonly in certain European populations, also got some attention a few years back for providing resistance to HIV infection.